The real problem with uCs with internal ADCs is to keep the digital noise out of the analog.

That's what I meant with:

But I made the experience, that so many factors are badly influencing precision of this ADC, that for me this ADC was not very trustworthy...

There are even others: Rejection of noise and glitches on supply voltage is a big problem!
Another problem is, that microcontroller and other circuitry on board (!) should rest absolutely quiet, when making these conversions. Even if power supply decoupling is adequate, noise resulting from externally located switching digital circuitry would erode precision. And this is something, which can NOT fully supressed, even not with multilayer board design! You would need digital chips not showing any stray capacitance at inputs and outputs. Everything happening on board entries micro and of course it's ADC via this insane stray capacitance at every input and output and erodes 0V potential of ADC.

So, how well it will work, depends on how quiet your board is, when conversions have to be done. By the way, that's also true for external ADC...

Kai